Study On The Influence Mechanism Of Straw Interlayer On Deep Soil Organic Carbon Sequestration And Its Stability In Saline Soil Of Hetao Irrigation Distinct

The Hetao Irrigation District is an important production base for high-quality agricultural products in China,but its secondary salinization problem is very prominent due to unreasonable irrigation.In recent years,the straw interlayer buried at 40 cm depth combination with irrigation has obvious effects on decreasing salt content in the cultivated layer and increasing yield.However,as the straw interlayer gradually decomposed,the dissolved organic carbon(DOC)produced was leached to the deep soil with irrigation,stimulating microbial activity and possibly affecting the stability of deep organic carbon.We had found that but the characteristics of soil organic carbon(SOC)accumulation in deep soil layers and its stability mechanism are not clear.Therefore,in this study was based on the field positioning experiment with different 0(CK),6(SL6),12(SL12)and 18(SL18)Mg·ha-1 dosages of straw interlayer in Hetao Irrigation District of Inner Mongolia from 2015 to 2021,combined with root box experiment and indoor culture experiment of 13C-glucose addition.The aim was to analyse the effects of straw interlayers on the carbon sequestration characteristics and CO2 emission characteristics in deep saline soil layers and to reveal the mechanisms of deep soil organic carbon stability in terms of physical,chemical and biological aspects.The main results are as follows:(1)The effect of different amounts of straw interlayers on the increase of SOC(ΔSOC)in 0-20 cm soil layer were mainly in 2015-2016,while the effect on ΔSOC in 60-100 cm deep layers were mainly in 2017-2019,and the ΔSOC in different soil layers increased with the increase of straw input amount.The SL12 and SL18 treatments increased the carbon sequestration efficiency(CSE)of the deeper soil layers,suggesting that the deeper soil layers could act as an important soil carbon sink under the straw interlayer measure.In addition,the straw interlayer provided soil nutrients and improved the salt environment.The results of structural equation model showed that the increase of pH and nutrient contents were conducive to CSE in 0-40 cm topsoil layer,while the decrease of pH and nutrient contents were conducive to CSE in 40-100 cm deep soil layers.(2)The CO2 concentration in the 0-100 cm soil profile increased with increasing soil depth after complete decomposition of straw interlayers,while the CO2 emission flux gradually decreased with increasing soil depth.In particular,the cumulative CO2 emission flux under SL18 was-0.8 g CO2-C·m-2 in the 80-100 cm soil layer,indicating that the high amount of straw interlayer promoted CO2 fixation in deeper soil layer.Structural equation modelling showed that CO2 emission fluxes in the 0-80 cm layers of saline soil were mainly regulated by soil environmental factors,while CO2 emission fluxes in the deep soil layer(80-100 cm)were mainly directly influenced by the amount of straw interlayer due to the stable soil environment.(3)After complete decomposition of straw interlayers,the formation of large aggregates(>0.25 mm)in deep layers of saline soil and the oxidation stability of SOC in aggregates were improved,and the process of soil large agglomeration was closely related to the improvement of oxidation stability of SOC.The results of redundancy analysis showed that the process of saline soil macro-aggregation and the oxidative stability of SOC in micro-aggregates under straw interlayers were the main explanatory factors for the variability in SOC storage of deep layers,and that the effect of physical structure stability was greater than that of oxidative stability of SOC.(4)The initial burial of the straw interlayer increased the DOC content of the 0-40 cm soil layers(1.7%-11.2%)and the microbial biomass carbon(MBC)content of the 0-60 cm soil layers(10.4%224.9%),but reduced the MBC content of the 60-100 cm soil layers(30.6%-83.6%)and increased the hydrolytic enzyme activity associated with carbon turnover.Correlation and redundancy analyses results showed that the increase of DOC content in the 0-40 cm soil layers were mainly due to the increase in Leucine aminophthalase(Lue)activity and the decrease in Peroxidase(PEO)activity,while the decrease in MBC content in the 40-100 cm deeper soil layers were due to the decrease in Xylanase(Xyl),Phenol oxidase(PhoOx)and PEO activity.Using soils from 40-60 cm and 60-80 cm after complete decomposition of straw interlayers with the addition of exogenous glucose.The results showed that the cumulative mineralisation of SOC and the priming effect in deep soil layers were reduced under straw interlayers.Further,the strength of the priming effect was depended mainly on the higher nutrient stoichiometry ratios(DOC/Avail-N,DOC/Avail-P and DOC/Avail-K)rather than on the characteristics of the in situ soil microbial community.In summary,straw interlayer measure had great potential to enhance SOC storage in deep soil layers and its stability in terms of physical structure of soil aggregates,SOC oxidation activity,and soil biology.By neglecting changes in deep soil carbon,previous studies may have underestimated the contribution of straw compartments to soil carbon sequestration,especially under high amounts of straw deep burial conditions,which transformed deep soils from a carbon source to a carbon sink and reduced SOC mineralisation.This study may provide a theoretical basis and technical guides for the enhancement of soil fertility and amelioration in saline soils in arid regions.